Indole (2,3-benzopyrole; FIG. 1 below) is a naturally occurring compound in plants and microorganisms. The indole ring system is found primarily in the amino acid tryptophan (2-amino-3-(3'-indolyl)-proprionic acid; FIG. 2 below), in plant hormones such as heterauxin (indole-3-acetic acid), in condensed ring alkyloids derived from tryptamine (2-(3'-indolyl)-ethylamine), and in complex monoterpene derived alkyloids such as strychnine and resetpine. ##STR1##
The present market for indole is estimated to be approximately 1/2 million pounds. Indole derivatives are used primarily as fragrance enhancers and as intermediates in the manufacture of pharmaceuticals and dyes. Substantial growth in the market for indole would occur if there were demand for the compound as a starting material for industrially produced L-tryptophan, the latter being an essential amino acid useful as a dietary supplement for livestock feeds. However, at an estimated volume of 370,000 pounds, tryptophan is now used only in specialty applications. Its high cost will not support penetration into the potentially large tryptophan formulated feed markets.
Although tryptophan cannot be economically extracted from plant sources, it can be biosynthesized at relatively low cost by enzymatic condensation of L-serine and indole catalyzed by tryptophan synthetase in a free cell fermenter. Unfortunately, in the absence of any commercial process for manufacturing indole, supplies of indole for use in this synthesis must be obtained exclusively and at rather high cost from the 240.degree.-260.degree. C. fraction of coal tar distillate. The present price of indole therefore prevents biosynthesized tryptophan to make a substantial penetration into the formula feed market. Thus, need presently exists for an inexpensive commercial synthesis for indole which will open up the market for the use of tryptophan as an affordable dietary supplement in animal feeds.
Another potential route to the compound indole involves dehydrocyclization of o-ethylaniline. Unfortunately, commercial iron oxide and platinum on alumina catalysts elicit short lifetimes and poor regenerability when used for this reaction. Dehydrocyclizations of o-ethylaniline over iron oxide fall below 40 percent conversion after only 50 hours. Although platinum on alumina initially gives high conversion and selectivity, for this reaction conversion and selectivity decreased rapidly with time on stream. For example, conversion using a platinum on alumina catalyst decreased from 100 percent at 20 hours to 59 percent at 50 hours with a corresponding decrease in selectivity from 91 percent to 74 percent. Also, the platinum on alumina catalyst had poor regenerability with a sharp decline in activity and longevity after the first two regenerations. It is thus particularly desired to obtain a cyclization catalyst capable of converting o-ethylaniline to indole where such catalyst has an extended lifetime, high conversion and selectivity, and excellent regenerability when compared to commercial iron oxide or platinum on alumina catalysts which are inadequate for dehydrocyclization of o-ethylaniline.
The compound benzofuran, like indole, is also subject to limited availability and high price due to the lack of a commercial synthesis for the material. Presently, benzofuran is available only as a product of coal tar extract. This fact limits its use and discourages research into wider applications. Benzofuran, and benzofuran derivatives, have applications in coumarin-indene resins for floor tile, synthetic rubbers, and as bonding agents for composites.